1. Field of the Invention
The present invention relates to a base metal composition for forming an external electrode of a multilayer electronic component such as a multilayer capacitor, for example, and more particularly, it relates to a base metal composition for an external electrode containing Cu as a main component.
2. Description of the Background Art
The material for an internal electrode of a multilayer capacitor is generally prepared from Ag or Ag-Pd. However, such an internal electrode material is high-priced. Therefore, an attempt has been made on employment of Ni, being a base metal, as a material which is at a lower cost.
On the other hand, an external electrode of such a multilayer capacitor is structured by forming a second electrode layer by plating Ni on a first electrode layer of Ag having excellent conductivity which can be baked at a low temperature, and further forming a third electrode layer of Sn or solder for improving solderability.
However, when the internal electrode is made of Ni, it and Ag are not solidly soluble with each other. When the internal electrode is made of Ni, therefore, it may be impossible to attain desired electrostatic capacity since the internal electrode cannot be reliably connected with the first electrode layer, which is made of Ag, forming the external electrode.
Therefore, attention is drawn to an external electrode consisting of Cu, which is entirely solidly soluble with respect to Ni. However, Cu is so easily oxidized that it is necessary to bake Cu conductive paste under a reducing atmosphere in order to prepare an external electrode. Under the reducing atmosphere, however, an organic substance which is contained in the Cu paste is decomposed at a low speed due to low oxygen concentration. Thus, glass contained in the Cu paste is melted, leading to foaming of a gas which is generated by decomposition of an organic substance incorporated in the melted glass upon further progress in sintering of Cu. Consequently, a phenomenon called blistering may be caused by such foaming, to partially swell the external electrode. When such blistering takes place, conductivity of the external electrode is reduced while it may not be possible to attain sufficient bond strength when the external electrode is electrically connected with another element or conductive pattern.
The aforementioned blistering may also be caused by reaction between ceramics and glass.
Japanese Patent Laying-Open No. 4-95307 (1992) proposes that the aforementioned blistering can be suppressed by employing Cu paste containing 5 to 15 percent by weight of lead zinc borosilicate glass frit and 2 to 5 percent by weight of zinc borosilicate glass frit having a crystallization temperature of 600.degree. to 750.degree. C.
When the Cu paste described in the above prior art is baked at a temperature exceeding 800.degree. C. in a reducing atmosphere, however, the surface of the external electrode may be partially covered with glass due to the considerably large blending ratios of the glass frit materials, and hence solder wettability may be reduced or it may not be possible to form a plating layer on the external electrode.